Different detector systems are known for detecting x-ray radiation. Scintillation detectors are widely used in order for example to enable flux densities of x-ray radiation occurring in the field of computed tomography to be measured. Scintillation detectors initially convert x-ray radiation photochemically into light quanta which have an energy suitable for enabling for example the light quanta to be detected with the aid of a semiconductor diode (photodiode).
For the purpose of computed tomography applications, efforts are furthermore directed toward the use of what are termed direct-conversion semiconductor detector elements, which absorb x-ray radiation in the semiconductor material without prior energy conversion. In the process, so-called electron-hole pairs are generated in the semiconductor detector element. It should be emphasized that the term “direct-conversion”, within the scope of the present invention, does not restrict the type of absorption of x-ray quanta in the semiconductor material. Although the description suggests a different inference, “direct-conversion semiconductor detector elements” enable both direct and indirect absorption of x-ray quanta (photon-assisted absorption). What matters with regard to the term “direct-conversion semiconductor material” is that an x-ray quantum is absorbed in the semiconductor material, in other words, in contrast to a scintillation detector, the roundabout route by way of a prior photochemical conversion of the x-ray radiation is avoided.
A certain quantity of free charge carriers are generated in the semiconductor detector element as a function of the energy of the absorbed x-ray radiation. In the process, a normally bound electron of the valence band of the semiconductor, upon absorption of x-ray radiation, gains at least so much energy that it is able, as mentioned, directly or indirectly to overcome the band gap of the semiconductor material used and in the conduction band of the semiconductor can contribute in an effectively “freely mobile” manner (the corresponding transport mechanisms in the semiconductor are known to the person skilled in the art) toward the conduction of a current. In the valence band there remains behind a vacant electron position, also referred to as a hole, which is likewise “mobile” in the valence band, which means that the generated electron vacancy can also contribute toward the conduction of a current. However, the drift or diffusion velocity may differ radically between electrons and holes.
If the freely moving charge carriers are brought into the area of influence of an electric field—for example by way of field electrodes that are connected to the semiconductor detector element, and by applying a voltage—then a photocurrent results owing to the availability of the free-moving charge carriers. By evaluating the pulse shape of the charge carrier packets (in particular the pulse height) it is possible to determine the number and the energy of the absorbed x-ray quanta or, as the case may be, of the absorbed x-ray radiation.
The drift and diffusion critical to the charge transport of the mobile charge carriers in the semiconductor, and hence to the pulse shape, are described by way of the movability (mobility p) of the free charge carriers. In particular the drift is also dependent in this case on the already mentioned electric field.
In particular it is aimed to use directly converting semiconductor detector elements based on CdTE, CdZnTe, CDZnTeSe, CdMnTe, InP, TlBr2, Hgl2. However, a disadvantage with these detector materials is that the electric field in the semiconductor material, and consequently the pulse shape of the photocurrent, can vary therein in an undesirable manner. In timescales relevant to the detection of x-ray radiation, these materials have unwanted numbers of stationary defects, called “traps”. These traps can intercept free-moving electrons of the conduction band or holes of the valence band and bind them in a stationary manner to the defects for a certain time. Furthermore, in the occupied or unoccupied state, the defects represent space charges. This formation of space charges is referred to as the polarization effect, as polarization for short, of the semiconductor detector element.
A disadvantageous aspect of the described effects is that the formation of space charge zones due to the traps or also the charge carrier trapping varies with respect to time as a function of the number of unoccupied or occupied traps. The electric field in the semiconductor material and the resulting pulse shape of the photocurrent may therefore be dependent on the temporal distance between absorption events, with the result that under certain conditions identical absorption events are not evaluated in a reproducible manner and a phenomenon called count rate drift occurs. In other words, the count rate of x-ray quanta for a temporally constant radiation density varies with time. Consequently, under certain conditions no unequivocal back-calculation to energy or number of absorbed x-ray quanta is possible, which means that a considerable amount of time and effort is required in order to make these detectors suitable for reliable use in imaging applications, such as in computed tomography for example.
In order to mitigate the cited polarization effects, and in particular to attenuate the time-dependent variation in the polarization during the detection of x-ray radiation, the semiconductor detector element can be irradiated.
The polarization can be varied when the defects are occupied by a corresponding charge carrier, but also when an unoccupied defect is generated. For this purpose a light source can be used, the radiation of which generates charge carriers in the semiconductor which can then be bound to the defect over a relatively long period of time. Such a defect is also referred to as a saturated defect, which, in contrast to an ionized defect, can be considered virtually charge-neutral. As a result the formation of space charge zones is varied, and in particular these can also be stabilized. The semiconductor detector element can be conditioned by this means such that an unequivocal back-calculation to the energy or count rate is possible.
In order to enable a reliable, unequivocal detection of x-ray radiation, in particular for imaging applications, it is furthermore necessary that the conditioning is likewise effected unequivocally, i.e. that the semiconductor detector element has a defined conditioning.